1. Field of the Invention
The present invention is directed to a method and system for cooling a plasma processing system, and in particular to a method and system for utilizing: (1) coolants that are expanded through an orifice and converted to gas that is applied to an exterior of a process tube and (2) an electrostatic shield that cools the processing tube by evaporating coolant using evaporation orifices in the electrostatic shield and directing the cooled vapor onto the process tube.
2. Description of the Background
In order to fabricate semiconductor wafers with submicron features using etch and deposition processes, modem semiconductor processing systems utilize plasma assisted techniques such as reactive ion etching (RIE), plasma enhanced chemical vapor deposition (PECVD), sputtering, reactive sputtering, and ion assisted vapor deposition (PVD). In addition to the above-referenced co-pending applications, another example of a gas plasma processing system is described in U.S. Pat. No. 5,156,345, to Wayne L. Johnson, the inventor of the present application. In such known systems, a gas is introduced to a processing environment wherein a gas plasma is formed and maintained through the application of radio frequency (RF) power. Typically, RF power is inductively coupled to the plasma using a helical coil.
Normally, the generation of a gas plasma also produces a substantial amount of heat that must be removed in order to maintain the processing system at a process-specific temperature. The removal of this heat has heretofore been inefficient and based on a cumbersome design. Known ESRF plasma sources have been cooled using baths of liquid coolants, such as FLUORINERT, which also act as dielectrics. The definition of a good dielectric at radio frequencies is that the fluid must have a low power loss per unit volume when exposed to an intense electric field. However, these particular fluids disadvantageously adsorb large quantities of gas, such as air. The four main sources of adsorbed gas are: (1) gas already trapped in the liquid prior to shipment (i.e., gas that was adsorbed prior to receipt by the plasma processing system user), (2) gas adsorbed into the liquid when the liquid is exposed to air, e.g., during pouring between containers before using the liquid, (3) gas adsorbed when air in the chamber is replaced by fluid during an initial filling cycle, and (4) the presence of air in any part of the system when fluid is being pumped. Furthermore, gas may be adsorbed into the coolant after the coolant pumps are stopped. When the pumps stop, if coolant in the high parts of the system drains to lower parts, then air replaces the drained coolant. When the pumps are restarted, the air may be broken down into bubbles which become another source of adsorbable gas.
In the high field regions, strong dissipation can occur leading to high local heating, hence, raising the local temperature of the coolant fluid. In so doing, the rate of gas evolution is increased permitting more gas to come out of solution, and generate bubbles that coalesce on the coil surface by dielectro-fluoretic attraction. The attached bubbles generate a dielectric difference at the coil surface which leads to non-uniform electric fields, localized heating, and arcing. This arcing can occur at voltages well below the measured dielectric strength of the fluid if the gas is not evolved from the liquid coolant before use in the resonator cavity. For example, FLUORINERT adsorbs a volume of gas equivalent to its own liquid volume and must be treated to remove the trapped gas.
In order to avoid arcing due to the rapid evolution of adsorbed gas, known systems gradually increase power to the plasma source while continuously pumping coolant through the ESRF plasma chamber. The gradual increase in RF power takes place over a period of time sufficient to slowly evolve adsorbed gas from the coolant. Although running the coolant in this way evolves trapped gases, a considerable amount of time is required. Often this process will take hours, thereby delaying the use of the plasma system in processing wafers.
In addition to the lengthy time period required by known systems to evolve adsorbed gas, the cooling systems coupled to a plasma source may be very cumbersome due to the large cooling lines used in large wafer (i.e., 300 mm) processing systems. Consequently, significant amounts of air are generally adsorbed when the processing chamber has been opened with the coolant lines remaining attached. The lines have typically remained attached since the coolant lines may contain hundreds of pounds of coolant. As a result, lifting the attached lines to open the chamber has been difficult, but not impossible.
Previously, it was not known how to replace the large lines with an alternate cooling mechanism. The large lines were required in order to provide the large coolant exchange (e.g., approximately 50-75 gallons/minute) needed to remove the heat from the process tube. Also, flexible lines were difficult to use because of the weight and pressure of the coolant required.
It is an object of the present invention to provide an improved method and system for cooling an ESRF source.
It is another object of the present invention to provide an improved method and system for cooling an ESRF source using a vapor coolant instead of a bath of liquid coolant.
It is a further object of the present invention to provide an ESRF cavity that can be tuned in atmospheric conditions instead of tuning using elements submerged in a temperature controlled fluid.
These and other objects of the present invention are achieved by a method and system utilizing coolant that is evaporated as it passes through a shield (prior to being applied to the exterior of a process tube) to remove the heat generated in an RF powered plasma source. Using a series of nozzles to apply low pressure coolant to the process tube, the present invention removes heat by vaporizing the liquid coolant and then pumping away the vapor. This method avoids the arcing that occurs in systems using baths of liquid coolant. Further, since the dielectric constant of the material around the coil remains close to the same between air and the dielectric fluid, the ESRF cavity can be tuned in air and will remain tuned over a wide range of temperatures. (That is, it is possible to reduce the shift in tuning that would otherwise result from a temperature-based change in the dielectric constant.)
More specifically, these and other objects of the present invention are achieved by a method and system utilizing a process tube that is cooled using a shield which surrounds the process tube. By expanding the coolant through a series of expansion orifices (e.g., disposed along the ribs of the shield) to a pressure lower than the coolant""s vapor pressure, the coolant is vaporized as it exits the orifices. That vapor is then impinged upon the process tube to remove heat from the shield and the process tube. Heat is removed through one or more of (1) forced convection of cool vapor over the surface of the process tube and (2) conduction.